This invention relates to a wiring board for packaging electronic component parts such as semiconductor integrated circuit devices and, particularly, to a multilayer wiring board intended for fast pulse signal propagation on the board.
FIG. 6 shows an example of the conventional multilayer wiring board structure, in which the board is made of a single insulation material, e.g., glass-epoxy (with a relative permittivity of 5), and notably the insulators 51, 52 and 53 interposed between the signal line layers 5 and 6 and the ground (power) line layers 4 and 7 are made of the same insulation material 50.
A conceivable method for improving the signal propagation characteristics of a wiring board is the use of a low relative permittivity material for the insulation layers of the board. Notably, fluorocarbonic polymer such as teflon (trademark of Dupont Corporation) has extremely small relative permittivity of approximately 2 as compared with other materials, and it is a very desirable material from the viewpoint of electrical characteristics. Application of teflon to the insulation material of the wiring board has been reported in an article entitled "Finstrate: A New Concept in VLSI Packaging" by Arun K. Malhotra, et al., in Hewlett-Packard Journal, August 1983.
However, many of low relative permittivity materials are disadvantageous to the use as insulation material for wiring boards in aspects other than the electrical characteristics. For instance, the abovementioned fluorocarbonic polymer has a low glass transition temperature, lacks in the dimensional stability, and is difficult in fine patterning of lines. It also has a large thermal expansion coefficient which is greatly different from that of IC chips mounted on the board, imposing an increased possibility of thermal fatigue in the terminal section.